Change Origin and rotate the frame of a mobile robot
select the body, then shift+s, cursor to selected, shift+a, empty-axis, select axis-empty, ctrl+i, then select axis-empty again, right mouse button, parent, object.  This parents to the body position where the axis is and all items are parented to the axis, but are moveable individually.  Move the axis for scale, rotation of all, or just the individual parts.
save a new file doing this to not lose work changed from.  Then add bones BEFORE all this, to selected areas using car rigging methods (search for appropriate online).  You rotate the bones added to the wheels and so forth, that you also parent to the RIG-axis-empty created.
to position all at the world origin, after you have made an axis parent, select the axis, then shift+s, cursor to world origin, then shift+s, selection to cursor.